A lubricant is a substance introduced to reduce friction between moving surfaces. It may also have the function of transporting foreign particles. The property of reducing friction is known as lubricity. Lubricant is a material that reduces the friction arising due to the sliding / gliding / rolling / moving of two different components in a machinery. Friction is an undesirable process that results in (i) wear and tear of the surfaces of the moving parts (ii) loss of enormous energy as heat dissipation (iii) lowered efficiency of the moving parts and (iv) damage of machine parts as seizure etc. The purpose of lubricant is to reduce the friction and minimize the frictional effects such as overheating / seizure of the components thereby reducing the losses. Also, the lubricant enables the smooth, flawless ejection of the moulded articles from the basic mould structure. The process of reducing the friction and wear between the two relatively moving components of the machinery is termed as lubrication.
Function of lubricants: The important functions of a lubricant are (i) to reduce wear and tear of the surfaces of the moving parts (ii) to reduce the loss of enormous energy as heat dissipation (iii) to increase the efficiency of the moving parts and (iv) to reduce damage of machine parts as seizure, surface deformation etc (v) to prevent the expansion of metal due to local frictional heat (vi) to minimize the possibility of corrosion, as the direct contact of metals is avoided by the formation of the lubricant film and (vii) to enhance smooth motion of the moving parts. Mechanism of lubrication
Three important types / modes of lubrication are (i) thick film lubrication or hydrodynamic lubrication (ii) thin film or boundary lubrication and (iii) extreme pressure lubrication. Hydrodynamic lubrication: In this, the two moving parts are separated by a thick film of lubricant, about 1000°A thick. This type of lubrication occurs in machine parts of low load and high speed such as in clocks, sewing machines (delicate instruments). The coefficient of friction is low, 0.01 to 0.003 Boundary lubrication: It occurs in machine parts of high load and low speed. Here thick film of lubricant cannot be maintained between the moving surfaces but the lubricants are adsorbed physically / chemically on the metal surfaces. The lubricant film thickness is as low as 2-3-molecule thickness with the frictional coefficient being 0.05 to 0.15. Extreme pressure lubrication: This mechanism occurs under conditions of high load and high speed. Under these conditions, the lubricant may vaporize / decompose due to local heat. Special additives called extreme pressure additives are used with lubricants to overcome this difficulty. Organic compounds containing active groups such as chlorine, phosphorous, sulphur are used as extreme pressure additives. At high temperatures, the additives react with metals giving metallic chlorides / sulphides / phosphides possessing high melting points. A good lubricant possesses the following characteristics:
* High boiling point.
* Low freezing point.
* High viscosity index.
* Thermal stability.
* Corrosion prevention.
* High resistance to oxidation.
One of the single largest applications for lubricants, in the form of motor oil, is protecting the internal combustion engines in motor vehicles and powered equipment. Typically lubricants contain 90% base oil (most often petroleum fractions, called mineral oils) and less than 10% additives. Vegetable oils or synthetic liquids such as hydrogenated polyolefins, esters, silicones, fluorocarbons and many others are sometimes used as base oils. Additives deliver reduced friction and wear, increased viscosity, improved viscosity index, resistance to corrosion and oxidation, aging or contamination, etc. Lubricants such as 2-cycle oil are added to fuels like gasoline which has low lubricity. Sulfur impurities in fuels also provide some lubrication...